Electrowinning is a known electrolytic technology used to recover metals from various aqueous, metal-containing solutions, i.e. electrolytes, e.g., the primary production of metal via leaching of ores or from electroplating rinse waters. A typical electrowinning system typically comprises three primary components: at least one electrolytic cell having a plurality of alternating anodes and cathodes, a source of DC electrical power (typically referred to as a “rectifier”), and a pump that pumps the electrolyte through at least one electrolytic cell between the anodes and cathodes. In a typical large electrowinning facility, tens of thousands of amperes of current at several hundred volts are passed through the electrolyte causing the metal to electrodeposit on the cathodes. Periodically, the cathodes are removed from the electrolyte and the electrodeposited metal is removed (“harvested”) and the cathodes replaced into the electrolyte. FIGS. 1A–1C show various aspects of typical electrowinning plates and cells and FIG. 2 shows a typical generic electrowinning system 20.
Referring now to FIGS. 1A–1C, a typical electrowinning cell 10 is shown schematically. The cell 10 comprises a container 11 (“cell”) for containing the electrolyte 12 and a plurality of cathodes 14 (shaded in FIGS. 1A–1C) and anodes 15 (unshaded in FIGS. 1A–1C), alternatively spaced as shown, with the electrolyte flowing therebetween. The anodes 15 and cathodes 14 typically comprise a support having a conductor bar 16 (also known as a “lug” or an “ear”) that is typically in direct electrical connection with an electrolytic plate 17 (FIG. 1B). FIG. 1C shows schematically a four-cell electrowinning cell-line comprising four electrolytic cells 10a–10d, electrically interconnected by five copper bus bars 18a–18e. As known to those skilled in the art, the conductor bars 16 of the cathodes 14 and anodes 15 of adjacent cells are typically in direct electrical connection with each other via the bus bars 18. More specific to the four-cell cell-line in FIG. 1C, the conductor bars 16 of the anodes 15 in the first cell 10a are physically touching and thus directly electrically connected to the first bus bar 18a. The anodes 15 in the first cell 10a are in circuit communication with the cathodes 14 in the first cell 10a via the electrolyte (not shown in FIG. 1C). “Circuit communication” as used herein indicates a communicative relationship between devices. Direct electrical, electromagnetic, and optical connections and indirect electrical, electromagnetic, and optical connections are examples of circuit communication. Two devices are in circuit communication if a signal from one is received by the other, regardless of whether the signal is modified by some other device. For example, two devices separated by one or more of the following-amplifiers, filters, transformers, optoisolators, digital or analog buffers, analog integrators, other electronic circuitry, fiber optic transceivers, or even satellites-are in circuit communication if a signal from one is communicated to the other, even though the signal is modified by one or more intermediate devices. As another example, an electromagnetic sensor is in circuit communication with a signal if it receives electromagnetic radiation from the signal. As a final example, two devices not directly connected to each other, but both capable of interfacing with a third device, e.g., a CPU, are in circuit communication. Also, as used herein, voltages and values representing digitized voltages are considered to be equivalent for the purposes of this application, unless otherwise noted, and thus, unless otherwise noted, the term “voltage” as used herein refers to either a signal, or a value in a processor representing a signal, or a value in a processor determined from a value representing a signal. All the conductor bars 16 of the cathodes 14 in the first cell 10a are physically touching and thus directly electrically connected to the second bus bar 18b. Similarly, all the conductor bars 16 of the anodes 15 in the second cell 10b are physically touching and thus directly electrically connected to the second bus bar 18b. Thus, all the cathodes 14 in the first cell 10a are electrically connected to all the anodes 15 in the second cell 10b via the second bus bar 18b. This structure repeats for the second cell 10b, the third cell 10c, and the fourth cell 10d, ending with all the conductor bars 16 of the cathodes 14 in the fourth cell 10d physically touching and thus directly electrically connected to the fifth bus bar 18e. 
FIG. 2 shows an electrowinning (“EW”) direct current (“DC”) power supply 22 in circuit communication with a bank of electrolytic cells 24. The bank of electrolytic cells 24 in FIG. 2 comprises a plurality of electrolytic cells 26a–26n. The bank 24 is shown in FIG. 2 as comprising one string of electrolytic cells 26a–26n all connected in series (known as a “cell-line”). Although the bank 24 is shown as a single cell-line, the embodiments of the present invention are believed to apply to virtually any configuration of any number of electrolytic cells connected in virtually any configuration, e.g., numerous cell-lines in series and/or parallel. The electrolytic cells are typically of the type as shown in FIGS. 1A–1C having a plurality of anode plates spaced from a plurality of cathode plates, with the EW electrolyte in the spaces therebetween. The EW DC power supply 22, also referred to as an EW rectifier, generates a very high-current signal at a voltage output 30 relative to a ground 32 that is typically electrically connected to the ends of the bank 24 of cells 26. If the four-cell cell-line of FIG. 1C were used as the bank 24, the output 30 would be electrically connected to the first bus bar 18a and the ground 32 would be electrically connected to the last bus bar 18e. In a typical large EW application having multitudes of cells 26, the output of the EW DC power supply 22 can be hundreds of volts having a very high current on the order of 5000 amperes to 50,000 amperes or more. As known to those skilled in the art, the current, indicated as leaving the EW DC power supply 22 at 34 and returning to the EW DC power supply 22 at 35, passes through a circuit comprising voltage output 30, the bank of electrolytic cells 24, ground 32, and back to the EW DC power supply 22. As discussed above, inside each electrolytic cell 26, the current 34, 35 passes from a bus bar 18 to the anodes 15 (FIGS. 1A and 1C), through the electrolyte 12 from which metals are being deposited (FIG. 1A), to the cathodes 14, to the next bus bar 18 (FIG. 1C).
As known to those in the art, the plates 17 of the cathodes 14 and anodes 15 can be made of different materials, depending on various factors, such as the electrolyte and the electrodeposited metal. For example, lead alloy (e.g. Pb—Ca—Sn) anodes are typically used to electrowin copper from various copper-containing solutions. If particular materials, e.g., lead, are selected for the anode plates, a reverse current will be developed if the EW DC power supply 22 ceases providing sufficient voltage and current to maintain a forward current in the cells 26. This reverse current is the result of the electrochemical reduction of the lead oxide surface deposit formed on the lead anode in normal operation and the oxidation of the product metal, e.g. copper. In ordinary EW installations, the reverse currents are not harmful, although they do decrease the net efficiency for the production of metal and increase the contamination of the electrolyte by loosening the surface deposits on the lead anode, and are generally ignored. Recently, however, various electrocatalytically active coatings have been used on electrowinning anodes, e.g., the technology disclosed in U.S. Ser. No. 09/648,506 and U.S. Pat. No 6,139,705 to the assignee of the present invention, which is marketed and sold in the industry as the Mesh-on-Lead (MOL™) technology. These electrocatalytically active coatings are sensitive to reverse currents and include such coatings as platinum or other platinum group metals or they can be represented by active oxide coatings such as platinum group metal oxides, magnetite, ferrite, cobalt spinel or mixed metal oxide coatings. The mixed metal oxide coatings can often include at least one oxide of a valve metal with an oxide of a platinum group metal including platinum, palladium, rhodium, iridium and ruthenium or mixtures of themselves and with other metals. When anode plates using these electrocatalytically active coatings are used in the same EW system with more traditional anode plates that can generate a reverse current, the reverse current can severely and irreversibly damage the electrocatalytically active coatings. For example, when anode plates using platinum group metal oxide containing coatings (especially those with palladium) are placed in series electrical relationship with lead anodes, if the EW DC power supply 22 ceases generating the EW voltage at output 30, a reverse current will be generated of sufficient magnitude to severely and irreversibly damage the electrocatalytically active coating on the anodes.
There is a need, therefore, for various systems and methods for protecting anodes having electrocatalytically active coatings in electrowinning cells from damage caused by reverse currents.